1. Field of the Invention
The present invention relates to a method of forming highly oriented diamond films by microwave plasma chemical vapor deposition. The films are used for electronic devices such as transistors, diodes and sensors, or for electronic parts such as heat sinks, surface acoustic elements, X-ray windows, optical materials and coatings.
2. Description of the Related Art
Diamond is excellent in heat resistance, and has a wide band gap. It is an electrically insulating material but becomes a semiconducting material once doped with impurities. Moreover, it has a large breakdown voltage, a large saturated drift velocity, and a small dielectric constant. Diamond, having such excellent features, is expected to be a useful for electronic parts and electronic devices for high temperature, high frequency and high electric field applications.
To make full use of the excellent characteristics of diamond for electronic devices, it is important to synthesize a high quality single crystal diamond in which impurities are controlled. However, a single crystal diamond obtained by a prior art high temperature/high pressure synthesis is limited in terms of the size. It is also impossible add/eliminate impurity by controlled amount. Besides the above-described high temperature/high pressure synthesis, diamond may be formed by vapor-phase synthesis such as microwave CVD, thermal filament, dc plasma CVD or combustion process. The vapor-phase synthesis is advantageous in control of impurities. It is capable of forming a thin film of diamond and therefore is expected to be applied to synthesize diamond for use in electronic materials. Conventionally, only polycrystalline diamond with high density crystal boundaries had been obtained on a substrate made of a material other than diamond through vapor-phase synthesis. However, in recent years, a method that a highly oriented diamond film strongly following the crystal orientation of the substrate can be synthesized in vapor-phase on a single crystal silicon substrate (S. D. Wolter, B. R. Stoner and J. T. Glass, Applied Physics Letters, Vol. 62, pp. 1215-1217 (1993); and X. Jiang and C. P. Klages, Diamond and Related Materials, Vol. 2, pp. 1112-1113 (1993)) has been proposed. In this method, a substrate is exposed to plasma for a specified period of time while a negative dc electric field is applied to the substrate before the film formation, and then the usual synthesis is performed. This method exhibits a possibility to manufacture diamond films with large areas and high qualities which are applicable for electronic devices.
However, the method disclosed in the above-described references, has the following disadvantages:
(1) In this method, polycrystalline diamond with a low degree of orientation is first formed on a silicon substrate. The degree of orientation is then gradually enhanced along with the vapor-phase synthesis time. Accordingly, to achieve the characteristics used for electronic devices, the highly oriented film requires the thickness ranging from 20 to 50 .mu.m or more. To ensure the required film thickness, the highly oriented diamond film must be synthesized for 1 to 2 days. This time span causes disadvantages in increasing the cost. It is also difficult to obtain a thin (or the order of .mu.m to a fraction of .mu.m) layer of highly oriented diamond films using the prior art. PA1 (2) The prior art highly oriented diamond film significantly improves the quality compared with the conventional polycrystalline diamond film. However, it fails to perfectly remove grain boundaries. To be suitably used as an electronic material, a diamond film having a planarized surface and with the furthermore reduced grain boundaries must be formed by vapor-phase synthesis. PA1 (3) To strongly oriented &lt;100&gt; textured crystal planes, the conventional highly oriented diamond film is formed under the condition which is very different from that of the usual vapor-phase synthesis for diamond. Consequently, the highly oriented diamond film which is obtained contains high density defects at the grain boundaries and on the surface, thus exerting adverse effect on the electrical properties of the interior and the surface of the film.